1. Technical Field
The present invention relates to a microcapsule comprising a shell (which serves as a partition layer) and a liquid substance that is encapsulated in the shell. Furthermore, the present invention relates to a microcapsule comprising the capsule shell and a dispersion (dispersion liquid) that is encapsulated as the liquid substance in the capsule shell, wherein the dispersion includes a solvent and electrophoretic fine particles that are dispersed in the solvent. Also, the present invention relates to a sheet involving using the above microcapsules and an electrophoretic display involving using the above microcapsules.
2. Background Art
A microcapsule has hitherto been well known wherein the microcapsule comprises a shell (capsule shell) (which serves as a partition layer) and an oily liquid substance (including such as fine-particle dispersion besides simple solution) that is encapsulated in the shell. Their usefulness is recognized in various uses such as: various sheets (e.g. non-carbon paper, pressure-measuring films); electrophoretic displays; and agents for controlled release of such as medicines and agricultural chemicals.
Usually, because the capsule shell which is a partition of the microcapsule is very thin, there occurs a phenomenon such that the liquid substance (particularly, a liquid component), which is a core (nucleus) substance and also an encapsulated material, bleeds out of the capsule shell. It is such as the above agents for controlled release that positively utilize this phenomenon. On the other hand, for example, as to uses which need to firmly retain the encapsulated liquid substance (e.g. microcapsules for electrophoretic displays), the above bleeding-out causes disadvantages such as: peripheral contamination; deterioration of powder properties of the microcapsule; cohesion of microcapsules; and deterioration and inferiority of normal or firm adhesion of the microcapsule to such as base materials; and is therefore problematic. In addition, also as to the microcapsule utilized for such as the above agents for controlled release, the encapsulated state of the liquid substance needs to be retained on a definite level before the use (e.g. during the preparation, preservation, and transportation) of the microcapsule, setting aside during and after the use of the microcapsule. However, it has not been easy to properly adjust the degree of the prevention and inhibition of the bleeding-out with consideration given to circumstances of during the use of the microcapsule.
Thus, as means of solving these problems, there have already been disclosed: an art in which an interfacial polymerization method and an In-situ polymerization method are combined together to thereby provide the capsule shell with a double structure, thus preventing the bleeding-out (e.g. refer to JP-A-032560/1988 (Kokai)); and an art in which a PVA (polyvinyl alcohol) layer is formed at the boundary between the core substance and the capsule shell to thereby enhance the durability to the bleeding-out (e.g. refer to JP-A-048773/1984 (Kokai)). However, considering these arts from levels demanded from such as recent years' technical levels and product quality, then they cannot be said to be satisfactory solutions. In addition, these arts cannot be said to be easy in their operational procedures, either, and are therefore inferior also in the economical advantages and the productivity.
In addition, as to the microcapsule, usually, also in demand for various uses and technical fields (enhancement of such as softness, firm adhesion, and further, facility in electrically charging the particles inside the capsule), it is required that the capsule shell should be made very thin, similarly to the above. However, there has been a problem such that, in compensation therefor, it is impossible to obtain sufficient physical strength. In addition, for example, some of uses such as non-carbon paper and pressure-measuring films have an object to finally destroy the capsule shell to thus make the encapsulated liquid substance flow out. However, also as to these, for the prevention of a problem such that the destruction unfavorably occurs other than when needed, the microcapsule needs to be provided with appropriate physical strength. However, its adjustment has not been easy. Therefore, it has hitherto always been made a subject that, even in the case where the capsule shell is made very thin, the physical strength to external loads such as stirring and application of pressure such as during the preparation and preservation of the microcapsule or such as during and after the use of the microcapsule for various uses should easily be enhanced and adjusted. Furthermore, also from the viewpoint of recent years' technical levels, more excellent ones are demanded.